Optical glass

ABSTRACT

There is provided an optical glass with a high refractive index and a low dispersion having a refractive index (nd) of not less than 1.75 and an Abbe&#39;s number (νd) of not less than 35 where the image formation characteristic is hardly affected by changes in temperature of the using environment. SiO 2 , B 2 O 3  and La 2 O 3  are contained as essential components and the ratio of the constituting components are adjusted whereby an optical glass in which a product of α and β where α is an average linear expansion coefficient at −30 to +70° C. and β is an optical elasticity constant at the wavelength of 546.1 nm is not more than 130×10 −12 ° C. −1 ×nm×cm −1 ×Pa −1  is able to be achieved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.13/676,525 filed on Nov. 14, 2012 which is continuation of U.S. patentapplication Ser. No. 13/155,797 filed on Jun. 8, 2011 which iscontinuation of U.S. patent application Ser. No. 12/100,798, filed onApr. 10, 2008, which is a nonprovisional application of U.S. ProvisionalPatent Application No. 60/976,193, filed on Sep. 28, 2007, the entirecontents of which are incorporated herein by references.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical glass having a highrefractive index and a low dispersion where the refractive index (nd) isnot less than 1.75 and an Abbe's number (νd) is not less than 35 andalso relates to an optical element such as lens and prism prepared byutilizing this optical glass. More particularly, it relates to anoptical glass having a high refractive index and a low dispersion whichis suitable as prism and projection lens of optical instrumentsrepresented by projector and camera where a highly precise imageformation characteristic and also relates to an optical element and anoptical instrument prepared therefrom.

2. Description of the Related Art

There is a very high demand for glass having a high refractive index anda low dispersion as a material for optical elements such as variouskinds of lenses and, with regard to an optical glass where a refractiveindex (nd) is not less than 1.75 and an Abbe's number (νd) is not lessthan 35, various kinds of glass compositions represented by PatentDocuments 1 to 3 have been known.

In recent years, there has been a progress in digitization of opticalinstruments and making them precise and there has been a demand of highproperties for optical elements used for the instruments for thereproduction (projection) such as a projector and a projection TV aswell as for instruments for taking pictures such as digital camera andvideo camera. The properties are now not only covering thecharacteristic such as refractive index, Abbe's number and degree ofcoloration which have been demanded for optical glass already but alsocovering little variation in the characteristic under an actually usingenvironment and little environmental load during the manufacture ofoptical glass and the processing of optical elements.

With regard to a change in image forming characteristic under anactually using environment, it has been presumed to be as follows thatan optical element such as lens and prism is fixed by a jig in opticalinstruments and, when temperature of the using environment changes (suchas a change in temperature in the box or use under high temperature),thermal expansion of the optical element is resulted and, due to thedifference in its expansion coefficient from that of the fixing jig,stress is resulted in the optical element whereby double refraction isresulted in the optical element and image forming characteristicchanges.

As mentioned above, when the image formation characteristic designed byoptical constants such as a refractive index and an Abbe's numberobtained under predetermined temperature (mostly around roomtemperature) is not achieved in the actual using environment, there is adisadvantage that, upon the optical designing, the using environment isto be predicted and the design is to be conducted presuming thecomplicated variations in the characteristic.

When there are components having a high environmental load such as alead (Pb) compound or an arsenic (As) compound at the time ofmanufacturing the optical glass and processing of optical elements,there is a disadvantage that special measures are necessary for theprevention of diffusion of polluting substances into air and water.Further, when a rare mineral resource represented by tantalum (Ta) isused in large amounts, not only the production cost becomes high butalso cost and labor for recovery of the source are necessary.

With regard to an optical glass with a high refractive index and a lowdispersion containing no components having high environmental load inthe glass composition, various glass compositions represented by thepatent gazettes 1 to 3 are disclosed but no consideration has been donefor changes in image formation characteristic under an actually usingenvironment.

Patent Document 1: JP-A-2005-306732

Patent Document 2: JP-A-2002-284542

Patent Document 3: JP-A-2004-161506

Patent Document 4: JP-A-56-160340

Patent Document 5: JP-A-52-14607

Under such circumstances, the an object of the invention is to providean optical glass having a high refractive index and a low dispersionwhere the refractive index (nd) is not less than 1.75 and the Abbe'snumber (νd) is not less than 35 which is hardly affected by imageformation characteristic by changes in temperature of the usingenvironment without the use of large amounts of the components havinghigh environmental load and the rare mineral resources.

SUMMARY OF THE INVENTION

In order to achieve the above object, the present inventors haverepeatedly carried out intensive tests and studies and, as a result,they have found that, when SiO₂, B₂O₃ and La₂O₂ are made to contain asessential components and ratio of the constituting components isadjusted, an optical glass having a high refractive index and a lowdispersion by which the product of α and β (where α is an average linearexpansion coefficient within −30° C. to +70° C. and β is an opticalelasticity constant at the wavelength of 546.1 nm) is able to be madenot more than 130×10⁻¹²° C.⁻¹×nm×cm⁻¹×Pa⁻¹ is now able to be preparedwithout the use of large amounts of components having a highenvironmental load and rare mineral resources whereby the above objectis achieved and they have accomplished the invention. The constitutionwill be shown as follows.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(Constitution 1)

An optical glass which is characterized in that the product of α and βwhere α is an average linear expansion coefficient within −30° C. to+70° C. and β is an optical elasticity constant at the wavelength of546.1 nm is not more than 130×10⁻¹²° C.⁻¹×nm×cm⁻¹×Pa⁻¹, SiO₂ iscontained therein in an amount of more than 1.0% by mass and less than12.0% by mass on the basis of an oxide, B₂O₃ is contained in an amountof 8.0 to 35.0% by mass, the ratio of SiO₂/B₂O₃ in terms of % by mass ismore than 0 and less than 0.6 and La₂O₃ is contained in an amount of25.0 to 50.0% by mass.

(Constitution 2)

The optical glass according to the constitution 1, wherein the glass hasoptical constants within the rages where the refractive index (nd) is1.75 to 2.00 and the Abbe's number (νd) is 35 to 55.

(Constitution 3)

The optical glass according to the constitution 1 or 2, wherein theglass contains, on the basis of an oxide, 0.0 to 40.0% by mass of Gd₂O₃,0.0 to 15.0% by mass of Y₂O₃, 0.0 to 15.0% by mass of ZrO₂, 0.0 to 25.0%by mass of Ta₂O₅, 0.0 to 18.0% by mass of Nb₂O₅ and 0.0 to 10.0% by massof WO₃.

(Constitution 4)

The optical glass according to any of the constitutions 1 to 3, whereinthe glass contains, on the basis of an oxide,

0.0 to 0.1% by mass of GeO₂ and/or

0.0 to 1.0% by mass of Yb₂O₃ and/or

0.0 to 1.0% by mass of Ga₂O₃ and/or

0.0 to 1.0% by mass of Bi₂O₃

and does not contain a lead compound such as PbO and an arsenic compoundsuch as As₂O₃.

(Constitution 5)

The optical glass according to any of the constitutions 1 to 4, whereinthe product of α and β where α is an average linear expansioncoefficient within −30° C. to +70° C. and β is an optical elasticityconstant at the wavelength of 546.1 nm is not more than 100×10⁻¹²°C.⁻¹×nm×cm⁻¹×Pa⁻¹.

(Constitution 6)

The optical glass according to any of the constitutions 1 to 5, whereinthe product of α and β where α is an average linear expansioncoefficient within −30° C. to +70° C. and β is an optical elasticityconstant at the wavelength of 546.1 nm is not more than 90×10⁻¹²°C.⁻¹×nm×cm⁻¹×Pa⁻¹.

(Constitution 7)

The optical glass according to any of the constitutions 1 to 6, whereinthe ratio of (Ta₂O₅+Nb₂O₅+WO₃)/(Gd₂O₃+Y₂O₃) in terms of % by mass basedon the oxides is more than 0.05 and less than 1.30.

(Constitution 8)

The optical glass according to any of the constitutions 1 to 7, whereinthe glass contains

0 to 5.0% of Li₂O and/or

0 to 5.0% of Na₂O and/or

0 to 5.0% of K₂O and/or

0 to 5.0% of Cs₂O and/or

0 to 5.0% of MgO and/or

0 to 5.0% of CaO and/or

0 to 5.0% of SrO and/or

0 to 5.0% of BaO and/or

0 to 3.0% of TiO₂ and/or

0 to 3.0% of SnO₂ and/or

0 to 3.0% of Al₂O₂ and/or

0 to 5.0% of P₂O₅ and/or

0 to 10.0% of ZnO and/or

0 to 5.0% of Lu₂O₂ and/or

0 to 3.0% of TeO₂ and/or

0 to 2.0% of Sb₂O₂ and/or

0 to 3.0% of F

in terms of % by mass on the basis of an oxide.

(Constitution 9)

The optical glass according to any of the constitutions 1 to 8, whereinthe glass contains less than 2.0% by mass of ZnO on the basis of anoxide.

(Constitution 10)

The optical glass according to any of the constitutions 1 to 9, whereinthe glass contains less than 3.5% by mass of Y₂O₃ on the basis of anoxide.

(Constitution 11)

The optical glass according to any of the constitutions 1 to 10, whereinthe ratio of (ZrO₂+Ta₂O₅+Nb₂O₅)/(SiO₂+B₂O₃) in terms of % by mass on thebasis of an oxide is less than 1.00.

(Constitution 12)

The optical glass according to any of the constitutions 1 to 11, whereinthe glass contains less than 3.5% by mass of Y₂O₃ on the basis of anoxide, the ratio of (ZnO+Y₂O₃)/La₂O₃ in terms of % by mass on the basisof an oxide is more than 0 and less than 0.5 and the sum of ZrO₂+Nb₂O₅in terms of % by mass is more than 5.0% and less than 13.0%.

(Constitution 13)

An optical glass, characterized in that, the glass contains more than1.0% by mass and less than 10.0% by mass of SiO₂,

15.0 to 28.0% by mass of B₂O₃,

28.0 to 35.0% by mass of La₂O₃,

25.0 to 35.0% by mass of Gd₂O₃,

5.0 to 9.0% by mass of ZrO₂ and

0.1 to less than 2.0% by mass of ZnO and

0.0 to 6.0% by mass of Ta₂O₅ and/or

0.0 to 5.0% by mass of Nb₂O₅ and/or

0.0 to 1.0% by mass of Sb₂O₃ and/or

0.0 to less than 1.0% by mass of Al₂O₂

on the basis of an oxide where the sum of ZrO₂+Nb₂O₅ is more than 5.0%by mass to less than 13.0% by mass, the glass has optical constantswithin such ranges that the refractive index (nd) is 1.78 to 1.83 andthe Abbe's number (νd) is 44 to 48 and the product of α and β where α isan average linear expansion coefficient within −30° C. to +70° C. and βis an optical elasticity constant at the wavelength of 546.1 nm is notmore than 90×10⁻¹²° C.⁻¹×nm×cm⁻¹×Pa⁻¹.

(Constitution 14)

An optical element such as lens and prism where the glass mentioned inthe constitutions 1 to 13 is a mother material.

(Constitution 15)

An optical element such as lens and prism which is prepared by a reheatpress processing of the glass mentioned in the constitutions 1 to 14.

(Constitution 16)

An optical instrument such as camera and projector using the opticalelement and the optical substrate material prepared by the glassmentioned in claims 1 to 15.

As a result of adoption of the above-mentioned embodiments, it is nowpossible to provide an optical glass having a high refractive index anda low dispersion which is hardly affected by image formationcharacteristic caused by changes in temperature under the usingenvironment and has a refractive index (nd) of not less than 1.75 and anAbbe's number (νd) of not less than 35.

The optical glass of the invention will be illustrated as follows.

The optical glass according to the above constitution 1 is characterizedin that the product of α and β where α is an average linear expansioncoefficient within −30° C. to +70° C. and β is an optical elasticityconstant at the wavelength of 546.1 nm is not more than 130×10⁻¹²°C.⁻¹×nm×cm⁻¹×Pa⁻¹ and the index of α×β shows changed amount of imageformation characteristic under the using environment. To be morespecific, it means that the more the average linear expansioncoefficient α, the more the expansion rate (changes in volume) of anoptical element against the changes in temperature under the usingenvironment and, therefore, it means that a big thermal stress isgenerated in an optical element fixed by a jig or the like. It alsomeans that the more the optical elasticity constant β, the more thedouble refraction generated by the resulted thermal stress and, in otherwords, it suggests that the less the α×β, the less the changes in theimage formation characteristic under the using environment.

Incidentally, there is an advantage that, when the product of α and β isnot more than 130×10⁻¹²° C.⁻¹×nm×cm⁻¹×Pa⁻¹, an image formationcharacteristic desired upon an optical design is apt to be achieved evenwhen the temperature changes under an actually using environment.

In order to achieve that the product of α and β is not more than130×10⁻¹²° C.⁻¹×nm×cm⁻¹×Pa⁻¹ in the optical glass having a highrefractive index and a low dispersion, the constitution 1 ischaracterized in that SiO₂ is contained therein in an amount of morethan 1.0% by mass and less than 12.0% by mass on the basis of an oxide,B₂O₃ is contained in an amount of 8.0 to 35.0% by mass, the ratio ofSiO₂/B₂O₃ in terms of % by mass is more than 0 and less than 0.6 andLa₂O₃ is contained in an amount of 25.0 to 50.0% by mass.

Now each of the components will be illustrated. An SiO₂ componentpromotes stable formation of glass and has an effect of suppressing thedevitrification (production of crystalline products) and cord(non-uniformity inside the glass) which are unfavorable for opticalglass. However, when it is contained too much, a refractive index (nd)is apt to become small and an optical elasticity constant β is apt tosignificantly increase and, as a result, a desired characteristic ishardly achieved. Therefore, its upper limit is less than 12.0% by mass,more preferably 11.5% by mass and, most preferably 11.0% by mass and theamount is preferably more than 1.0% by mass, more preferably not lessthan 1.2% by mass and, most preferably, not less than 1.4% by mass.Although the SiO₂ component is able to be contained in any materialform, it is preferred to be introduced in a form of an oxide (SiO₂),K₂SiF₆ or Na SiF₆.

A B₂O₃ component promotes a stable glass formation the same as the SiO₂component does and it is an inevitable component for achieving a smallaverage linear expansion coefficient. However, when its amount is toosmall, stable glass is hardly resulted while, when its amount is toomuch, a refractive index (nd) is apt to become small and an opticalelasticity constant β tends to significantly increase whereby thedesired characteristic is hardly available. Its upper limit ispreferably 35% by mass, more preferably 34% by mass and, mostpreferably, 33% by mass while its lower limit is preferably 8.0% bymass, more preferably 8.5% by mass and, most preferably, 9.0% by mass.The B₂O₃ component is able to be contained therein in a material formsuch as H₃BO₃, Na₂B₄O₇, Na₂B₄O₇.10H₂O or BPO₄ and it is preferred to beintroduced in a form of H₃BO₃.

As a result of making the ratio of SiO₂/B₂O₃ less than 0.6 in terms of %by mass, not only an effect of increasing the fusing property of thematerial and the stability of glass is achieved but also an effect ofsuppressing the increase of an average linear expansion coefficient α isachieved. When it exceeds the upper limit, it may happen that theaverage linear expansion coefficient α increases and further that meltedresidue (hardly-fusible crystals mostly containing SiO₂) upon fusion ofthe glass is generated whereby the productivity becomes bad and theinner quality is badly affected. More preferred range of % by mass is0.03 to 0.59 and most preferred one is within a range of 0.05 to 0.58.

In addition to the effect of enhancing the refractive index and makingthe dispersion small (making the Abbe's number large), the La₂O₃component also has an action of making the optical elasticity constant βsmall. However, if it is contained too much, glass becomes significantlyunstable and is apt to be devitrified. Accordingly, its upper limit ispreferably 50% by mass, more preferably 49.5% by mass and, mostpreferably, 49.0% by mass while its lower limit is preferably 25% bymass, more preferably 25.5% by mass and, most preferably, 26% by mass.The La₂O₃ component is able to be contained therein in any material formand it is preferred to be introduced thereinto in a form of an oxide(La₂O₃) and a nitrate or a nitrate hydride (La (NO₃)₃.xH₂O where x isany integer).

The optical glass according to the above constitution 2 is characterizedin having optical constants of such ranges where a refractive index (nd)is 1.75 to 2.00 and an Abbe' s number (νd) is 35 to 55 and it is usefulfor various optical elements and optical designs.

The above-mentioned optical constants are useful in an optical designparticularly because miniaturization of an optical system is possible(The characteristic of a high refractive index that a refractive indexis not less than 1.75 is able to afford a big refractive amount of lighteven in the case of a thin lens and the low dispersing characteristicthat the Abbe's number is not less than 35 is able to make the shift offocus (chromatic aberration) small even in the case of a single lens.).

In the optical glass of the above constitutions 1 and 2, a Gd₂O₃component gives an effect of making the refractive index high and makingthe dispersion small the same as an La₂O₃ component does but, when it iscontained too much, devitrification is apt to happen the same as in thecase of the La₂O₃ component. Accordingly, its upper limit is preferably40% by mass, more preferably 39% by mass and, most preferably, 38% bymass . The Gd₂O₃ component is able to be contained therein in anymaterial form and it is preferred to be introduced thereinto in a formof an oxide (Gd₂O₃) or a fluoride (GdF₃).

Although an Y₂O₃ has an effect of adjusting the refractive index and thedispersion, there is a risk that the desired optical constants are notachieved if it is contained too much. Its upper limit is preferably 15%by mass, more preferably 14.5% by mass and, most preferably, 14.0% bymass. The Y₂O₃ component is able to be contained therein in any materialform and it is preferred to be introduced thereinto in a form of anoxide (Y₂O₃) or a fluoride (YF₃).

Although there is no particular technical disadvantage if the range ismentioned as above, its amount is preferred to be less than 3.5% by masswhen a production cost is taken into consideration because Y₂O₃ is therarest mineral resource among the components which are able to achievethe characteristics of a high refractive index and a low dispersion.

A ZrO₂ component has an effect of enhancing the refractive index (nd)and improving the resistance to devitrification but, since the ZrO₂component is a hardly fusing component, fusing at high temperature isforced in the manufacture of glass if it is contained too much and theloss in energy is apt to cause a problem. On the other hand, there aresome cases where an effect of suppressing the devitrification isachieved when a predetermined amount is contained. Accordingly, itsupper limit is preferably 15% by mass, more preferably 13% by mass and,most preferably, 12% by mass while its lower limit is preferably 1% bymass, more preferably 2% by mass and, most preferably, 3% by mass. Whenno devitrification is resulted in the glass even if no ZrO₂ component isadded, the component may not be added. The ZrO₂ component is able to beintroduced in any material form and it is preferred to be introduced ina form of an oxide (ZrO₂) or a fluoride (ZrF₄).

Since a Ta₂O₅ component has an effect of enhancing the refractive indexto stabilize the glass, it may be optionally contained. However, theTa₂O₅ component is a rare mineral resource, has a high material price,is a hardly melting component and forces a fusion at high temperature inthe manufacture of glass whereby it has a characteristic that not onlyproduction cost increases but also an optical elasticity constant βincreases. Accordingly, the upper limit of its content is preferably 25%by mass. More preferred upper limit is 22% by mass and the mostpreferred upper limit is 19% by mass. Although the Ta₂O₅ component isable to be introduced in any material form, it is preferred to beintroduced in a form of an oxide (Ta₂O₅).

An Nb₂O₅ component has an effect of increasing the refractive index andstabilizing the glass the same as a Ta₂O₅ component does and it may beoptionally contained within a range of 0 to 18% by mass. However, theNb₂O₅ component is a hardly melting component and forces a fusion athigh temperature in the manufacture of glass whereby it has acharacteristic that not only production cost increases but also anoptical elasticity constant β increases. Accordingly, the upper limit ofits content is preferably 18% by mass. More preferred upper limit is 16%by mass and the most preferred upper limit is 14% by mass. Although theNb₂O₅ component is able to be introduced in any material form, it ispreferred to be introduced in a form of an oxide (Nb₂O₅).

A WO₃ component has an effect of adjusting the refractive index and thedispersion and of improving the resistance of the glass todevitrification. However, when it is contained too much, coloration ofthe glass is significant and transmittance particularly in the visibleto short wave regions (shorter than 500 nm) becomes low and that is notpreferred. Accordingly, its upper limit is preferably 10% by mass, morepreferably 8% by mass and, most preferably, 6% by mass. Although the WO₃component is able to be introduced in any material form, it is preferredto be introduced in a form of an oxide (WO₃).

In the optical glass of the above constitution 4, a GeO₂ component maybe optionally added within a range of 0.0 to 0.1% by mass for adjustmentof the refractive index and for adjustment of viscosity of the fusedglass. However, sine it is a rare mineral resource and is expensive, itis preferred not to be contained at all. Although each of Yb₂O₃, Ga₂O₃and Bi₂O₃ may be optionally added for adjustment of the refractiveindex, it has a property of increasing the optical elasticity constant βand, therefore, its upper limit is 1.0% by mass. However, since thosecomponents are also rare mineral resources, more preferred upper limitis 0.5% by mass and, most preferably, nothing is added at all. Each ofGeO₂, Yb₂O₃, Ga₂O₃ and Bi₂O₃ components may be introduced in anymaterial form and it is preferred to be introduced in a form of an oxide(GeO₂, Yb₂O₃, Ga₂O₃ and Bi₂O₃).

Since a lead compound such as PbO and an arsenic compound such as As₂O₃are components having high environmental loads, it is preferred not tobe contained at all except in the case of unavoidable mixing-in.

In the optical glass according to the above constitutions 5 and 6, theproduct of α and β is preferably not more than 100×10⁻¹²°C.⁻¹×nm×cm⁻¹×Pa⁻¹ and, most preferably, not more than 90×10⁻¹²°C.⁻¹×nm×cm⁻¹×Pa⁻¹ for utilizing in optical elements of higher precisionand higher definition.

When the value of α×β is smaller, an image forming characteristic in theactually using environment becomes faithful to the optically designedvalue calculated on the basis of the optical property near the roomtemperature and, therefore, there is an advantage that it is notnecessary to conduct a complicated optical simulation with a presumptionof various using environments.

In the optical glass of the constitution 7, the ratio of(Ta₂O₅+Nb₂O₅+WO₃)/(Gd₂O₃+Y₂O₃) in terms of % by mass which is the ratioin terms of % by mass of the total amount of Ta₂O₅, Nb₂O₅ and WO₃ havinga strong effect of enhancing the dispersion to the total amount of Gd₂O₃and Y₂O₃ having an effect of reducing the dispersion is made within arange of more than 0.05 and less than 1.30 whereby the desired Abbe'snumber (35 to 55) is apt to be achieved, so the above range ispreferred. More preferably, it is within a range of 0.055 to 1.29 and,most preferably, it is within a range of 0.06 to 1.28.

When the components within the mentioned range are contained in theoptical glass of the constitution 8, the characteristics mentioned inthe constitution 1 to 7 are able to be stably achieved. Reasons forlimitation of each component will be illustrated as follows.

Since the alkali metal oxide components (Li₂O, Na₂O, K₂O and Cs₂O) givean effect of enhancing the fusing property of the glass, it may beoptionally contained but, when they are contained too much, it is apt tohappen that an average linear expansion coefficient a increases or arefractive index lowers whereby the glass becomes unstable and undesiredphenomenon such as devitrification is resulted and, accordingly, each ofthem is preferred to be made within a range of 0.0 to 5.0% in terms of %by mass. More preferred upper limits are 4.5% for an Li₂O component, anNa₂O component and a K₂O component and 4.0% for a Cs₂O component . Themost preferred upper limit for an Li₂O component is 2.0% and, withregard to the components of Na₂O, K₂O and Cs₂O, they are not containedat all. Although the alkali metal oxide components may be introduced invarious forms such as a carbonate (Li₂CO₃, Na CO₃, K₂CO₃ and Cs CO₃), anitrate (LiNO₃, NaNO₃, KNO₃ and CsNO₃), a fluoride (LiF, NaF, KF andKHF₂) and a complex salt (Na₂SiF₆ and K₂SiF₆), it is preferred to beintroduced in a form of a carbonate and/or a nitrate.

Alkali earth metal oxide components (MgO, CaO, SrO and BaO) give aneffect of adjusting the refractive index and the optical elasticityconstant of the glass and, therefore, they are able to be optionallycontained but, if they are contained too much, desired optical constants(particularly, a refractive index) are apt to be hardly achieved wherebyeach of them is preferred to be contained within a range of 0.0 to 5.0%in terms of % by mass. More preferred upper limit is 4.0% for the MgOcomponent and the CaO component and 4.5% for the SrO component and theBaO component. The most preferred upper limit is that no MgO componentis contained at all and is 3.0% for the CaO component and 4.0% for theSrO component and the BaO component. Although the alkali earth metaloxide components may be introduced in various forms such as a carbonate(MgCO₃, CaCO₃ and BaCO₃), a nitrate (Sr(NO₃)₂ and Ba(NO₃)₂) and afluoride (MgF₂, CaF₂, SrF₂ and BaF₂), it is preferred to be introducedin a form of a carbonate and/or a nitrate and/or a fluoride.

The TiO₂ component is able to be optionally contained for adjustment ofa refractive index and an Abbe's number but, when it is containedexcessively, coloration of the glass is apt to become significant and,particularly, transmittance of the visible short wavelength (500 nm andshorter) tends to become bad. Accordingly, its preferred upper limit is3.0% by mass, the more preferred upper limit is 2.5% by mass and themost preferred upper limit is 2.0% by mass. Although the TiO₂ componentmay be introduced in any material form, it is preferred to be containedin a form of an oxide (TiO₂).

An SnO₂ component gives an effect of suppressing the oxidation of thefused glass and making it clear and of preventing the worsening oftransmittance to irradiation of light and, therefore, it may beoptionally contained. However, if it is contained excessively, there isa risk of coloration of the glass due to reduction of fused glass and ofgiving an alloy with the fusing device (particularly, noble metal suchas Pt). Its upper limit is preferably 3.0% by mass, more preferably 2.0%by mass and, most preferably, 1.0% by mass. Although the SnO₂ componentmay be introduced in any material form, it is preferred to be introducedin a form of an oxide (SnO and SnO₂) or a fluoride (SnF₂ and SnF₄).

An Al₂O₂ component is able to give an effect of enhancing the chemicaldurability of the optical glass and the optical element and of improvingthe resistance of the fused glass to devitrification and, therefore, itmay be optionally contained. However, if it is contained excessively, itis apt to happen that a refractive index significantly lowers and anoptical elasticity constant becomes too big. Accordingly, its upperlimit is preferably 3.0% by mass, more preferably 2.0% by mass and, mostpreferably, 1.0% by mass. Although the Al₂O₂ component may be introducedin any material form, it is preferred to be introduced in a form of anoxide (Al₂O₂), a hydroxide (Al(OH)₂) or a fluoride (AlF₂).

A P₂O₅ component gives an effect of improving the fusing property of theglass and, therefore, it may be optionally contained but, when it iscontained too much, it is apt to happen that resistance of glass todevitrification becomes significantly bad and an optical glass having nodevitrification is hardly available. Accordingly, its upper limit ispreferably 5.0% by mass, more preferably 1.0% by mass and, mostpreferably, it is not contained at all. Although the P₂O₅ component maybe introduced in any material form, it is preferred to be introduced ina form of Al(PO₃)₃, Ca(PO₃)₂, Ba(PO₃)₂, BPO₄ or H₃PO₄.

A ZnO component has an effect of improving the fusing property of theglass and also making an average linear expansion coefficient a smalland, therefore, it is able to be optionally contained within a range of0 to 10.0% by mass. Since it has a property of significantly increasingthe optical elasticity constant β, a desired characteristic is apt to behardly available if it is contained excessively. The more preferredrange is less than 5.0% by mass and the most preferred range is lessthan 2.0% by mass. Preferably, the lower limit is 0.1% by mass. Althoughthe ZnO component may be introduced in any material form, it ispreferred to be introduced in a form of an oxide (ZnO) and/or a fluoride(ZnF₂).

An Lu₂O₃ component gives an effect of achieving a high refractive indexand a low dispersion the same as the La₂O₃, Gd₂O₃ and Y₂O₃ components doand, therefore, it is able to be optionally contained within a range of0 to 5.0% by mass. However, since it is a rare mineral resource, it isnot preferred to contain excessively. Its upper limit is more preferably3.0% by mass and, most preferably, it is not contained at all. Althoughthe Lu₂O₃ is able to be introduced in any material form, it is preferredto be introduced in a form of an oxide (Lu₂O₃).

A TeO₂ component gives an effect of promoting the clarifying action uponfusion of the glass and, therefore, it is able to be optionallycontained within a range of 0 to 3.0% by mass. However, when it iscontained excessively, coloration of the glass is significant andtransmittance is apt to become bad. More preferred upper limit is 1.5%by mass and, most preferably, it is not contained at all. Although theTeO₂ component is able to be introduced in any material form, it ispreferred to be introduced in a form of an oxide (TeO₂).

An Sb₂O₃ has an effect as a defoaming agent for glass and, therefore, itis able to be optionally contained within a range of 0 to 2.0% by mass.However, when it is contained more than the upper limit, there is a riskthat an excessive foaming is apt to happen upon fusion of the glass orit may form an alloy with a fusing device (particularly a noble metalsuch as Pt) and, therefore, it is preferred that more than the upperlimit is not contained. Although the Sb₂O₃ component may be introducedin any material form, it is preferred to be introduced in a form of anoxide (Sb₂O₃ and Sb₂O₅) or Na₂H₂Sb₂O₇.5H₂O.

An F component gives an effect of making the Abbe's number big or makingthe optical elasticity constant β small and, therefore, it is able to beoptionally contained within a range of 0 to 3.0% by mass. However, whenit is contained in an amount of more than the upper limit, there is arisk that a refractive index becomes low and an average linear expansioncoefficient a increases. More preferred upper limit is 2.8% by mass andthe most preferred upper limit is 2.5% by mass. The F component isintroduced into the glass when the material form is introduced in a formof a fluoride in the introduction of the above-mentioned various kindsof oxides.

The expression of amount of each component used in this specification onthe basis of an oxide means % by mass of the corresponding resultedoxide of each component to the total component with a presumption thatall of oxides, composite salts, metal fluorides, etc. used as materialsof the constituting components of the glass of the invention aredecomposed upon fusion to convert into oxides and, in the case of afluoride, mass of the actually contained fluorine atoms to the mass ofthe resulting oxide is expressed in terms of % by mass.

Various kinds of transition metal components such as V, Cr, Mn, Fe, Co,Ni, Cu, Ag and Mo except Ti are colored even when each of them iscontained in small amount either solely or jointly whereby absorption inthe specific wavelength in visible region is resulted. Therefore, in thecase of an optical glass using the wavelength of visible region, it ispreferred that they are not substantially contained. Further, there is atendency that the use of Pb, Th, Cd, Tl, As, Os, Be and Se components isdecreasing as a harmful chemical substance and it is necessary to takean action in view of environmental measure not only in the manufacturingsteps for glass but also in processing steps and disposal after makinginto a product. Consequently, it is preferred that those components arenot substantially contained when much importance is to be paid toenvironmental influence.

In the optical glass of the above-mentioned constitution 11, the ratioof the total amount of ZrO₂, Ta₂O₅ and Nb₂O₅ which are hardly-fusingcomponent to the total amount of SiO₂ and B₂O₃ which are glass-formingcomponents, i.e. (ZrO₂+Ta₂O₅+Nb₂O₅)/(SiO₂+B₂O₃) is made less than 1.00in terms of % by mass whereby there is achieved an effect that there isno necessity of making the glass fusing temperature significantly highand consumption of energy is able to be reduced. When the above ratio ismore than 1.00, since each of Ta₂O₅ and Nb₂O₅ components is a raremineral resource, there is a risk that, the more the ratio, the higherthe material cost and further that the amount of the glass-formingcomponents becomes relatively small whereby glass becomes unstable.Moreover, there is a risk that the relative amount of ZrO₂, Ta₂O₅ andNb₂O₅ which increase the optical elasticity coefficient becomes high andthat of B₂O₃ which has an effect of lowering the average linearexpansion coefficient a whereby the product of α and β increases. Thus,the above is not preferred for the production of the desired opticalglass cheaply.

In the optical glass of the above constitution 12, the Y₂O₃ componentwhich is the rarest mineral resource among the components which are ableto achieve the characteristics of the high refractive index and the lowdispersion is made less than 3.5% by mass whereby an effect of reducingthe manufacturing cost and producing the glass in a stable andever-lasting manner is able to be achieved. Further the ratio of(ZnO+Y₂O₂)/La₂O₂ in terms of % by mass is more than 0 and less than 0.5whereby an effect of stable formation of an optical glass giving thedesired production of α and β is able to be achieved. Furthermore, thesum of ZrO₂+Nb₂O₅ in terms of % by mass is more than 5.0% and less than13.0% whereby an effect of limiting the amount of the hardly fusingcomponents, suppressing the energy consumption and providing an opticalglass having an excellent resistance to devitrification is able to beachieved.

In the optical glass of the above constitution 13, the range of theconstitution component ratio in the most suitable optical glass amongthe above optical glass products of the above constitutions 1 to 12 ismade clear. To be more specific, the composition of the glass ismaintained to the following ones, i.e.

more than 1.0% by mass and less than 10.0% by mass of SiO₂,

15.0 to 28.0% by mass of B₂O₃,

28.0 to 35.0% by mass of La₂O₃,

25.0 to 35.0% by mass of Gd₂O₃,

5.0 to 9.0% by mass of ZrO₂ and

0.1 to less than 2.0% by mass of ZnO and

0.0 to 6.0% by mass of Ta₂O₅ and/or

0.0 to 5.0% by mass of Nb₂O₅ and/or

0.0 to 1.0% by mass of Sb₂O₃ and/or

0.0 to less than 1.0% by mass of Al₂O₃

whereby there is an advantage that an optical glass in which opticalconstants are within such ranges that the refractive index (nd) is 1.78to 1.83 and the Abbe's number (νd) is 44 to 48 and the product of α andβ where a is an average linear expansion coefficient at −30° C. to +70°C. and β is an optical elasticity constant at the wavelength of 546.1 nmis not more than 90×10⁻¹²° C.⁻¹×nm×cm⁻¹×Pa⁻¹ is able to be stablyprepared. When the constituting components and the amounts thereof aremade within a ratio of the predetermined range as such, the use ofhardly fusing components and rare mineral resources is suppressed to aminimum extent and production of an optical element for the use of highprecision and high definition where changes in image formingcharacteristic under the using environment is little is now possiblewithout the use of the components having a high load on environments.

As mentioned in the constitutions 14 to 16, the optical glass mentionedin the above constitutions 1 to 13 is useful as a mother material forthe preparation of optical elements such as lenses and prisms and, whenthe optical elements are utilized for cameras and projectors, imageformation and projection characteristic with high precision and highdefinition are able to be achieved.

Since the composition is expressed in terms of % by mass in the glasscomposition of the invention, it is unable to be directly expressed interms of mol %. However, the composition of each of the components interms of mol % existing in the glass composition satisfying variouscharacteristics demanded in the invention has almost the followingvalues.

As to the range for the constitution 1, it is 2.0 to 25.0 mol % forSiO₂, 25 to 65 mol % for B₂O₃, more than 0 to less than 0.7 in terms ofthe molar % ratio for SiO₂/B₂O₃ and 10 to 30 mol % for La₂O₃.

As to the range for the constitution 3, it is 0 to 18 mol % for Gd₂O₃, 0to 10 mol % for Y₂O₃, 0 to 10 mol % for ZrO₂, 0 to 10 mol % for Ta₂O₅, 0to 10 mol % for Nb₂O₅ and 0 to 5 mol % for WO₃.

As to the range for the constitution 4, it is 0.0 to 0.1 mol % for GeO₂,0.0 to 1.0 mol % for Yb₂O₃, 0.0 to 1.0 mol % for

Ga₂O₃ and 0.0 to 1.0 mol % for Bi₂O₃.

As to the range for the constitution 7, the ratio in terms of mol % for(Ta₂O₅+Nb₂O₅+WO₃)/(Gd₂O₃+Y₂O₃) is more than 0.03 and less than 1.25.

As to the range for the constitution 8, that in terms of mol % is asfollows.

0 to 7.0% for Li₂O,

0 to 5.0% for Na₂O,

0 to 5.0% for K₂O,

0 to 3.0% for Cs₂O,

0 to 5.0% for MgO,

0 to 5.0% for CaO,

0 to 5.0% for SrO,

0 to 5.0% for BaO,

0 to 5.0% for TiO₂,

0 to 3.0% for SnO₂,

0 to 3.0% for Al₂O₃,

0 to 3.0% for P₂O₅,

0 to 7.0% for ZnO,

0 to 2.0% for Lu₂O₃,

0 to 1.0% for TeO₂,

0 to 1.0% of Sb₂O₃ and

0 to 10% for F.

As to the range for the constitution 9, it is less than 5.0 mol % forZnO.

As to the range for the constitution 10, it is less than 4.0 mol % forY₂O₃.

As to the range for the constitution 11, the mol % ratio is less than0.8 for (ZrO₂+Ta₂O₅+Nb₂O₅)/(SiO₂+B₂O₃).

As to the range for the constitution 12, it is less than 4.0 mol % forY₂O₃, the mol % ratio is more than 0 and less than 1.0 for (ZnO+Y₂O₃)/La₂O₃ and the mol % sum is more than 5.0% and less than 13.0% for(ZrO₂+Nb₂O₅).

As to the range for the constitution 13, it is 3 to 22 mol % for SiO₂,27 to 63 mol % for B₂O₃, 10 to 25 mol % for La₂O₃, 6 to 15 mol % forGd₂O₃, 4 to 10 mol % for ZrO₂, 0.1 to 2.0 mol % for ZnO, 0 to 5.0 mol %for Ta₂O₅, 0 to 3 mol % for Nb₂O₅, 0 to 0.5 mol % for Sb₂O₃ and 0 toless than 1.0 mol % for Al₂O₃.

EXAMPLES

Now the invention will be illustrated in more detail by way of thefollowing Examples although the invention is not limited to thoseExamples.

Tables 1 to 8 show glass composition, refractive index (nd), Abbe'snumber (νd), average linear expansion coefficient α at −30 to +70° C.,optical elasticity coefficient β at the wavelength of 546.1 nm, productof α and β and ratio and sum of the amounts of various components forExamples (1 to 38) which are suitable for the production of opticalglass with a high refractive index and a low dispersion where therefractive index (nd) is not less than 1.75 and the Abbe's number (νd)is not more than 35 in which image formation characteristic is hardlyaffected by changes in the temperature in the using environment.

Table 9 shows glass compositions and various properties of ComparativeExamples (A to C) for known optical glass products.

In this table, Comparative Example A is Example 6 in JP-A-2005-306732,Comparative Example B is Example 1 in JP-A-2002-284542 and ComparativeExample C is Example 7 in JP-A-2004-161506. The refractive indexes (nd)and Abbe's numbers (νd) in the table are those mentioned in each of theabove gazettes.

For the optical glass prepared, its refractive index (nd), Abbe's number(νd), average linear expansion coefficient (α) at −30° to +70° C. andoptical elasticity coefficient (β) at the wavelength of 546.1 nm weremeasured as follows.

(1) Refractive index (nd) and Abbe's number (νd)

Measurements were conducted for the optical glass where the temperaturelowering rate with a gradual cooling was made −25° C/hour.

(2) Average linear expansion coefficient (a) at −30 to +70° C.

Measurement was conducted in accordance with the method mentioned in thestipulations by the Japan Optical Glass Industry Association (JOGIS16-2003) (a method for the measurement of average linear expansioncoefficient of optical glass at about ambient temperature). As a testpiece, a sample of 50 mm length and 4 mm diameter was used.

(3) Optical elasticity constant (β) at the wavelength of 546.1 nm

An optical elasticity constant (β) was determined in such a manner thatthe shape of a sample was made into a disk of 25 mm diameter and 8 mmthickness after subjecting to a face-to-face polishing, a compressingload was applied in the predetermined direction, an optical pathdifference generated in the center of the glass was measured andcalculation was conducted according to the formula δ=β.d.F. A super-highvoltage mercury lamp was used as a light source for the measurement at546.1 nm. In the above formula, optical path difference, glass thicknessand stress are given as δ (nm), d (cm) and F (Pa), respectively.

TABLE 1 % by mass 1 2 3 4 5 SiO₂ 2.62 2.60 2.60 2.60 2.60 B₂O₃ 31.2031.22 31.22 29.22 30.21 Al₂O₃ P₂O₅ Y₂O₃ 11.70 10.70 10.70 10.70 1.75La₂O₃ 44.31 45.31 44.31 45.31 45.19 Gd₂O₃ Yb₂O₃ Lu₂O₃ 1.00 TiO₂ ZrO₂6.60 6.60 6.60 6.50 6.60 SnO₂ TeO₂ 0.10 Nb₂O₅ 1.62 1.62 1.62 1.62 2.71Ta₂O₅ WO₃ ZnO 0.90 0.90 0.90 0.90 0.90 MgO CaO SrO 1.00 1.00 1.00 1.001.00 BaO Li₂O Na₂O 1.00 K₂O Cs₂O Sb₂O₃ 0.05 0.05 0.05 0.05 0.04 F 1.00Total 100.00 100.00 100.00 100.00 100.00 α 62 62 62 64 62 β 1.44 1.431.43 1.40 1.42 α × β 89.28 88.66 88.66 89.60 88.04 nd 1.772 1.773 1.7731.773 1.780 νd 49.6 49.6 49.6 50.0 48.5 SiO₂/B₂O₃ 0.084 0.083 0.0830.089 0.086 (Ta₂O₅ + Nb₂O₅ + 0.138 0.151 0.151 0.151 0.252 WO₃)/(Gd₂O₃ +Y₂O₃) (ZrO₂ + Ta₂O₅ + 0.243 0.243 0.243 0.255 0.284 Nb₂O₅)/(SiO₂ + B₂O₃)(ZnO + Y₂O₃)/La₂O₃ 0.284 0.256 0.262 0.256 0.258 ZrO₂ + Nb₂O₅ 8.22 8.228.22 8.12 9.31

TABLE 2 % by mass 6 7 8 9 10 SiO₂ 2.00 2.60 2.60 2.32 2.60 B₂O₃ 29.0029.20 29.20 27.87 28.20 Al₂O₃ P₂O₅ 0.54 Y₂O₃ 8.80 10.80 10.80 La₂O₃29.49 47.06 45.06 39.02 45.06 Gd₂O₃ 26.97 18.89 Yb₂O₃ Lu₂O₃ TiO₂ 0.50ZrO₂ 6.71 6.60 6.60 6.66 6.60 SnO₂ TeO₂ 0.05 0.04 Nb₂O₅ 2.96 3.30 3.803.75 3.80 Ta₂O₅ 2.28 WO₃ 0.50 ZnO 0.90 0.90 0.95 0.90 MgO CaO SrO 1.001.00 0.50 BaO Li₂O 0.50 Na₂O K₂O 1.00 Cs₂O Sb₂O₃ 0.04 0.04 0.04 F Total100.00 100.00 100.00 100.00 100.00 α 60 61 61 61 61 β 1.46 1.38 1.401.42 1.40 α × β 87.60 84.18 85.40 86.62 85.40 nd 1.783 1.788 1.788 1.7881.789 νd 47.3 47.4 47.4 48.1 47.3 SiO₂/B₂O₃ 0.069 0.089 0.089 0.0830.092 (Ta₂O₅ + Nb₂O₅ + 0.194 0.375 0.352 0.199 0.398 WO₃)/(Gd₂O₃ + Y₂O₃)(ZrO₂ + Ta₂O₅ + 0.385 0.311 0.327 0.345 0.338 Nb₂O₅)/(SiO₂ + B₂O₃)(ZnO + Y₂O₃)/La₂O₃ 0 0.206 0.260 0.024 0.260 ZrO₂ + Nb₂O₅ 9.67 9.9010.40 10.41 10.40

TABLE 3 % by mass 11 12 13 14 15 SiO₂ 2.60 2.30 2.00 2.00 2.00 B₂O₃29.00 27.87 26.54 26.54 26.64 Al₂O₃ 0.10 P₂O₅ Y₂O₃ 12.86 5.40 La₂O₃43.00 38.76 32.49 32.49 32.49 Gd₂O₃ 13.50 26.98 26.97 26.97 Yb₂O₃ Lu₂O₃TiO₂ ZrO₂ 6.60 6.66 7.29 6.71 6.71 SnO₂ TeO₂ Nb₂O₅ 3.80 3.38 3.56 2.963.56 Ta₂O₅ 1.14 0.28 WO₃ ZnO 1.10 0.45 1.00 2.00 1.00 MgO 0.50 CaO SrO1.00 BaO 0.68 Li₂O Na₂O K₂O Cs₂O Sb₂O₃ 0.04 0.04 0.04 0.05 0.05 F Total100.00 100.00 100.00 100.00 100.00 α 61 60 61 61 63 β 1.43 1.43 1.431.43 1.40 α × β 87.23 85.80 87.23 87.23 88.20 nd 1.789 1.796 1.799 1.8001.801 νd 47.3 47.0 46.3 47.0 46.7 SiO₂/B₂O₃ 0.090 0.083 0.075 0.0750.075 (Ta₂O₅ + Nb₂O₅ + 0.295 0.239 0.132 0.120 0.132 WO₃)/(Gd₂O₃ + Y₂O₃)(ZrO₂ + Ta₂O₅ + 0.329 0.371 0.380 0.349 0.360 Nb₂O₅)/(SiO₂ + B₂O₃)(ZnO + Y₂O₃)/La₂O₃ 0.325 0.151 0.031 0.062 0.031 ZrO₂ + Nb₂O₅ 10.4010.04 10.85 9.67 10.27

TABLE 4 % by mass 16 17 18 19 20 SiO₂ 2.15 2.00 2.04 2.00 7.49 B₂O₃26.54 26.54 26.54 26.54 17.71 Al₂O₃ P₂O₅ Y₂O₃ 5.00 La₂O₃ 32.49 32.4532.99 32.48 29.81 Gd₂O₃ 26.98 27.00 26.97 15.98 31.06 Yb₂O₃ Lu₂O₃ TiO₂ZrO₂ 6.71 6.72 6.72 6.71 7.04 SnO₂ 0.60 TeO₂ Nb₂O₅ 4.09 2.96 3.69 2.96Ta₂O₅ 2.28 2.28 2.25 WO₃ ZnO 1.00 1.00 1.00 MgO CaO SrO BaO 3.00 Li₂ONa₂O K₂O Cs₂O 1.00 Sb₂O₃ 0.04 0.05 0.05 0.05 0.04 F Total 100.00 100.00100.00 100.00 100.00 α 59 60 61 61 65 β 1.48 1.42 1.41 1.41 1.30 α × β87.32 85.20 86.01 86.01 84.50 nd 1.803 1.804 1.804 1.805 1.806 νd 45.946.6 46.6 46.5 47.4 SiO₂/B₂O₃ 0.081 0.075 0.077 0.075 0.423 (Ta₂O₅ +Nb₂O₅ + 0.152 0.194 0.137 0.250 0.072 WO₃)/(Gd₂O₃ + Y₂O₃) (ZrO₂ +Ta₂O₅ + 0.376 0.419 0.364 0.419 0.369 Nb₂O₅)/(SiO₂ + B₂O₃) (ZnO +Y₂O₃)/La₂O₃ 0.031 0 0.030 0.133 0.034 ZrO₂ + Nb₂O₅ 10.80 9.68 10.41 9.677.04

TABLE 5 % by mass 21 22 23 24 25 SiO₂ 7.49 7.51 7.52 7.52 7.50 B₂O₃17.71 18.33 18.02 17.94 17.76 Al₂O₃ P₂O₅ Y₂O₃ 3.00 La₂O₃ 29.81 32.4131.75 31.34 29.71 Gd₂O₃ 31.06 31.66 31.52 31.45 31.08 Yb₂O₃ Lu₂O₃ TiO₂ZrO₂ 7.04 7.06 7.07 7.07 7.05 SnO₂ 0.60 TeO₂ Nb₂O₅ 1.98 1.32 0.99 Ta₂O₅2.25 1.75 2.63 5.25 WO ₃ ZnO 1.00 1.00 1.00 1.01 1.00 MgO CaO 0.60 SrOBaO Li₂O Na₂O K₂O Cs₂O Sb₂O₃ 0.04 0.05 0.05 0.05 0.05 F Total 100.00100.00 100.00 100.00 100.00 α 64 64 63 64 63 β 1.36 1.29 1.32 1.26 1.37α × β 87.04 82.56 83.16 80.64 86.31 nd 1.812 1.814 1.816 1.816 1.816 νd47.4 46.6 46.6 46.6 46.6 SiO₂/B₂O₃ 0.423 0.410 0.417 0.419 0.422(Ta₂O₅ + Nb₂O₅ + 0.066 0.063 0.097 0.115 0.169 WO₃)/(Gd₂O₃ + Y₂O₃)(ZrO₂ + Ta₂O₅ + 0.369 0.350 0.397 0.420 0.487 Nb₂O₅)/(SiO₂ + B₂O₃)(ZnO + Y₂O₃)/La₂O₃ 0.134 0.031 0.031 0.032 0.034 ZrO₂ + Nb₂O₅ 7.04 9.048.39 8.06 7.05

TABLE 6 % by mass 26 27 28 29 30 SiO₂ 7.49 2.43 5.17 1.94 2.38 B₂O₃17.96 24.96 20.66 24.64 24.02 Al₂O₃ P₂O₅ Y₂O₃ 1.50 1.50 La₂O₃ 31.8138.46 37.63 41.88 42.90 Gd₂O₃ 31.05 17.57 21.31 16.34 12.26 Yb₂O₃ Lu₂O₃TiO₂ ZrO₂ 7.04 6.63 6.80 6.50 5.57 SnO₂ TeO₂ 0.100 Nb₂O₅ 3.00 4.98 3.635.55 7.40 Ta₂O₅ 2.92 4.24 2.92 WO ₃ 0.60 3.00 ZnO 1.00 0.50 0.51 1.00MgO CaO SrO BaO Li₂O Na₂O K₂O Cs₂O Sb₂O₃ 0.05 0.05 0.05 0.05 0.05 FTotal 100.00 100.00 100.00 100.00 100.00 α 63 62 63 64 62 β 1.34 1.361.29 1.34 1.31 α × β 84.42 84.32 81.27 85.76 81.22 nd 1.819 1.819 1.8251.826 1.834 νd 45.5 44.7 44.7 43.4 42.6 SiO₂/B₂O₃ 0.417 0.097 0.2500.079 0.099 (Ta₂O₅ + Nb₂O₅ + 0.116 0.414 0.369 0.523 0.750 WO₃)/(Gd₂O₃ +Y₂O₃) (ZrO₂ + Ta₂O₅ + 0.394 0.530 0.568 0.453 0.602 Nb₂O₅)/(SiO₂ + B₂O₃)(ZnO + Y₂O₃)/La₂O₃ 0.031 0.052 0.014 0.000 0.058 ZrO₂ + Nb₂O₅ 10.0411.61 10.43 12.05 12.97

TABLE 7 % by mass 31 32 33 34 35 SiO₂ 2.82 7.39 4.61 6.43 5.89 B₂O₃23.38 14.96 17.67 11.67 12.46 Al₂O₃ P₂O₅ Y₂O₃ 3.00 1.33 La₂O₃ 43.9239.99 41.96 42.11 40.00 Gd₂O₃ 8.17 15.38 12.08 13.54 15.38 Yb₂O₃ 1.00Lu₂O₃ TiO₂ 0.50 0.30 ZrO₂ 6.54 5.99 6.27 6.00 5.99 SnO₂ 0.50 0.33 TeO₂Nb₂O₅ 6.27 1.00 3.14 0.50 1.00 Ta₂O₅ 5.85 14.69 12.27 15.82 17.38 WO₃ZnO 1.90 1.00 MgO CaO SrO 0.30 BaO Li₂O Na₂O 0.50 K₂O 0.50 Cs₂O Sb₂O₃0.05 0.10 0.07 0.10 F Total 100.00 100.00 100.00 100.00 100.00 α 62 6264 65 67 β 1.31 1.38 1.31 1.31 1.33 α × β 81.22 85.56 83.84 85.15 89.11nd 1.835 1.847 1.859 1.878 1.881 νd 42.7 42.7 41.8 41.2 40.7 SiO₂/B₂O₃0.121 0.494 0.261 0.551 0.473 (Ta₂O₅ + Nb₂O₅ + 1.085 1.020 1.276 1.0981.195 WO₃)/(Gd₂O₃ + Y₂O₃) (ZrO₂ + Ta₂O₅ + 0.712 0.970 0.973 1.233 1.328Nb₂O₅)/(SiO₂ + B₂O₃) (ZnO + Y₂O₃)/La₂O₃ 0.068 0.000 0.000 0.077 0.025ZrO₂ + Nb₂O₅ 12.81 6.99 9.41 6.50 6.99

TABLE 8 % by mass 36 37 38 SiO₂ 6.39 5.92 6.42 B₂O₃ 11.96 12.50 11.65Al₂O₃ P₂O₅ Y₂O₃ 1.33 La₂O₃ 39.99 40.47 42.02 Gd₂O₃ 15.39 15.46 13.51Yb₂O₃ Lu₂O₃ TiO₂ 0.25 ZrO₂ 5.99 6.03 5.99 SnO₂ 0.50 0.50 TeO₂ Nb₂O₅ 1.001.00 0.50 Ta₂O₅ 18.68 17.62 15.59 WO ₃ 2.19 ZnO 0.65 MgO CaO SrO BaOLi₂O Na₂O K₂O Cs₂O Sb₂O₃ 0.10 0.10 0.10 F Total 100.0 100.0 100.0 α 6668 67 β 1.30 1.32 1.34 α × β 85.80 89.76 89.78 nd 1.883 1.883 1.883 νd40.8 40.8 40.7 SiO₂/B₂O₃ 0.534 0.474 0.551 (Ta₂O₅ + Nb₂O₅ + 1.279 1.2041.245 WO₃)/(Gd₂O₃ + Y₂O₃) (ZrO₂ + Ta₂O₅ + 1.399 1.338 1.233Nb₂O₅)/(SiO₂ + B₂O₃) (ZnO + Y₂O₃)/La₂O₃ 0 0.016 0.032 ZrO₂ + Nb₂O₅ 6.997.03 6.49

TABLE 9 Comp. Comp. Comp. % by mass Ex. A Ex. B Ex. C SiO₂ 6.70 1.006.00 B₂O₃ 10.80 24.00 34.50 Al₂O₃ P₂O₅ Y₂O₃ 3.80 2.18 La₂O₃ 41.80 40.6830.00 Gd₂O₃ 9.60 12.68 Yb₂O₃ Lu₂O₃ TiO₂ ZrO₂ 5.20 6.00 5.00 SnO₂ TeO₂Nb₂O₅ 1.30 7.75 Ta₂O₅ 15.90 WO₃ ZnO 4.50 5.75 22.00 MgO CaO 2.00 SrO BaO0.50 Li₂O 0.20 Na₂O K₂O Cs₂O Sb₂O₃ 0.20 F Total 100.0 100.4 100.0 α 7167 51 β 1.28 1.50 2.55 α × β 90.88 100.50 130.05 nd 1.88 1.834 1.783 νd40.9 42.7 47.7 SiO₂/B₂O₃ 0.620 0.042 0.174 (Ta₂O₅ + Nb₂O₅ + 1.284 0.522WO₃)/(Gd₂O₃ + Y₂O₃) (ZrO₂ + Ta₂O₅ + 1.280 0.550 0.123 Nb₂O₅)/(SiO₂ +B₂O₃) (ZnO + Y₂O₃)/La₂O₃ 0.199 0.195 0.733 ZrO₂ + Nb₂O₅ 6.50 13.75 5.00

All of the glass products in accordance with the invention mentioned inTables 1 to 8 were prepared in such a manner that common materials foroptical glass such as the corresponding oxide, hydroxide, carbonate,nitrate, fluoride, hydroxide, metaphosphate, etc. as the materials foreach component were used, weighed and mixed in a predetermined ratio,poured over into a platinum crucible, fused for 3 to 4 hours at thetemperature range of 1,200 to 1,400° C. in an electric furnace dependingupon the easiness of the fusion of the glass composition, stirred formaking uniform, lowered down to an appropriate temperature, placed in ametal mold or the like and gradually cooled.

It has been found that, as shown in Tables 1 to 8, all of the preferredExamples of the invention are able to achieve the desired opticalconstants and the product α×β. On the contrary, in the ComparativeExamples shown in Table 9, Comparative Example 1 is able to achieve arelatively small α×β but, as compared with Examples 36 to 38 where theoptical constants are similar, the mass % ratio of SiO₂/B₂O₃ exceeds 0.6whereby an average linear expansion coefficient a becomes big and theproduct α×β is more than 90×10⁻¹²° C.⁻¹×nm×cm⁻¹×Pa⁻¹. In ComparativeExample B, abundant ZnO is contained and, therefore, the opticalelasticity constant β becomes large and the product α×β exceeds100×10⁻¹²° C.⁻¹×nm×cm⁻¹×Pa⁻¹ as compared with Examples 30 to 32 wherethe optical constants are similar. Besides that, amount of SiO₂ is smalland the mass % ratio of SiO₂/B₂O₃ is less than 0.05 and, therefore,resistance of the glass against devitrification is not sufficient and,when the glass is cast, crystals are generated nearly on the wholesurface of the glass. In Comparative Example C, amount of ZnO issignificantly large and the mass % ratio of (ZnO+Y₂O₃)/La₂O₃ is as bigas 0.733 and, therefore, the optical elasticity constant β increases andthe product α×β exceeds 130×10⁻¹²° C.⁻¹×nm×cm⁻¹×Pa⁻¹.

When the glass products of the Examples mentioned in Tables 1 to 8 weresubjected to a cold processing or a reheat press processing, no problemsuch as devitrification was resulted but they were able to be stablymade into various lens and prism forms.

When the lens or the prism prepared as such was installed in a camera ora projector and an image formation characteristic was confirmed, theimage formation characteristic which is expected by an optical designutilizing the optical constants obtained at room temperature wasreproducible even upon operations at high temperature (about 50 to 70°C.).

Although the invention was illustrated in detail hereinabove for anobject of exemplification, it will be understood that the

Examples are merely for an object of exemplification and that variousmodifications are able to be carried out by persons skilled in the artwithout deviating from the idea and scope of the invention.

In accordance with the invention, there is provided an optical glasswith a high refractive index and a low dispersion having a refractiveindex (nd) of not less than 1.75 and an Abbe's number (νd) of not lessthan 35 where the image formation characteristic is hardly affected bychanges in temperature of the using environment and, when the opticalglass is used, lenses and prisms for image projecting (reproducing)instruments such as projectors and picture-taking devices such as highlyprecise camera are able to be stably manufactured.

What is claimed is:
 1. An optical glass comprising, by mass on oxidebasis: SiO₂ more than 1.0% and less than 12.0%, B₂O₃ 8.0 to 35.0%, theratio of SiO₂/B₂O₃ being more than 0 and less than 0.6, and La₂O₃ 25.0to 50.0%, wherein the product of α×β, where α is an average linearexpansion coefficient within −30° C. to +70° C. and β is an opticalelasticity constant at the wavelength of 546.1 nm, is not more than130×10⁻¹²° C.⁻¹×nm×cm⁻¹×Pa⁻¹.
 2. The optical glass according to claim 1,wherein the glass has optical constants within the rages where therefractive index (nd) is 1.75 to 2.00 and the Abbe's number (νd) is 35to
 55. 3. The optical glass according to claim 1, wherein the glassfurther comprises 0.0 to 40.0% of Gd₂O₃, 0.0 to 15.0% of Y₂O₃, 0.0 to15.0% of ZrO₂, 0.0 to 25.0% of Ta₂O₅, 0.0 to 18.0% of Nb₂O₅ and 0.0 to10.0% of WO₃.
 4. The optical glass according to claim 1, wherein theglass further comprises 0.0 to 0.1% by mass of GeO₂, 0.0 to 1.0% by massof Yb₂O₃, 0.0 to 1.0% by mass of Ga₂O₃, and 0.0 to 1.0% by mass of Bi₂O₂and the optical glass does not contain a lead compound such as PbO andan arsenic compound such as As₂O₂.
 5. The optical glass according toclaim 1, wherein the product of α×β is not more than 100×10⁻¹²°C.⁻¹×nm×cm⁻¹×Pa⁻¹.
 6. The optical glass according to claim 1, whereinthe product of α and β is not more than 90×10⁻¹²° C.⁻¹×nm×cm⁻¹×Pa⁻¹ . 7.The optical glass according to claim 1, wherein the ratio of(Ta₂O₅+Nb₂O₅+WO₃)/(Gd₂O₃+Y₂O₃) is more than 0.05 and less than 1.30. 8.The optical glass according to claim 1, wherein the glass contains 0 to5.0% of Li₂O, 0 to 5.0% of Na₂O, 0 to 5.0% of K₂O, 0 to 5.0% of Cs₂O, 0to 5.0% of MgO, 0 to 5.0% of CaO, 0 to 5.0% of SrO, 0 to 5.0% of BaO, 0to 3.0% of TiO₂, 0 to 3.0% of SnO₂, 0 to 3.0% of Al₂O₃, 0 to 5.0% ofP₂O₅, 0 to 10.0% of ZnO, 0 to 5.0% of Lu₂O₃, 0 to 3.0% of TeO₂, 0 to2.0% of Sb₂O₃, 0 to 3.0% of F.
 9. The optical glass according to claim1, wherein the glass contains less than 2.0% of ZnO.
 10. The opticalglass according to claim 1, wherein the glass contains less than 3.5% ofY₂O₃.
 11. The optical glass according to claim 1, wherein the ratio of(ZrO+Ta₂O₅+Nb₂O₅)/(SiO+B₂O₃) is less than 1.00.
 12. The optical glassaccording to claim 1, wherein the glass contains less than 3.5% of Y₂O₃,the ratio of (ZnO+Y₂O₃)/La₂O₃ is more than 0 and less than 0.5 and thesum of ZrO+Nb₂O₅ is more than 5.0% and less than 13.0%.
 13. An opticalglass, comprising, by mass on oxide basis: more than 1.0% and less than10.0% of SiO₂, 15.0 to 28.0% of B₂O₃, 28.0 to 35.0% of La₂O₃, 25.0 to35.0% of Gd₂O₃, 5.0 to 9.0% of ZrO₂ and 0.1 to less than 2.0% of ZnO 0.0to 6.0% of Ta₂O₅, 0.0 to 5.0% of Nb₂O₅, 0.0 to 1.0% of Sb₂O₃, and 0.0 toless than 1.0% by mass of Al₂O₃, wherein the sum of ZrO+Nb₂O₅ is morethan 5.0% to less than 13.0%, the glass has optical constants withinsuch ranges that the refractive index (nd) is 1.78 to 1.83 and theAbbe's number (νd) is 44 to 48 and the product of α×β, where a is anaverage linear expansion coefficient within −30° C. to +70° C., and β isan optical elasticity constant at the wavelength of 546.1 nm, is notmore than 90×10¹²° C.⁻¹×nm×cm⁻¹×Pa⁻¹.
 14. The optical glass according toclaim 1, wherein the optical glass is included in an optical element.15. The optical glass according to claim 1, wherein the optical elementis prepared by a reheat press processing.
 16. The optical glassaccording to claim 1, wherein the optical glass is included in anoptical instrument.